As the global village becomes smaller, and the use of satellite communications grows daily, efforts to push the capacity of satellites and their transmissions presents ever-increasing hurdles. In the area of satellite data communication, in which the high priorities are speed and accuracy, the unique paradigm of space-linked communications has variables that must be compensated for in order to achieve the maximum throughput of error-free data.
With satellites that are not geostationary, a significant variable that affects any satellite transmission system is the Doppler effect. This well known effect occurs when the distance between a transmitter and receiver of signals is changing. In the case of satellite transmissions, the effect manifests itself by shifting signals away from their intended frequency. Normally, all satellites in a particular orbit will have the same Doppler shift when in the same point in the orbit with respect to a point on the Earth. This shift changes with respect to a particular ground station as the satellite moves in its orbit. Both the shift for a given position and the rate of change may be generally calculated and compensated for in any communications system. However, many current satellite communication systems rely on multiple satellites simultaneously to provide necessary relative location information and to determine the Doppler shift.
There are also further variables that may contribute to signal frequency translation. For example, the satellite's internal oscillators may experience drift within (or perhaps even beyond) their tolerance. This drift may constantly or randomly change. In addition, satellite signals often experience further frequency translations caused by temperature. This effect depends upon the time day, as each satellite in a given orbital plane will enter and exit the earth's shadow at different times throughout the day.
These translation errors have made accurate location measurements difficult, if not impossible, when using only one satellite. With large-scale, high-cost systems, the luxury of providing multiple satellite access for location calculations is possible. With expanded satellite use, locating remote users accurately has become more critical due to various technical and economic reasons.
Such translation errors have had minimal impact in the past upon the proper functioning of satellite communications systems. With analog communications, including voice and video relays, slight frequency translations had little effect on the ultimate transmission. The frequency control loops of data modems could deal with a small and constant frequency translation error caused by the satellite's internal local oscillators. However, relatively large constantly varying frequency offsets due to Doppler caused by low orbiting satellites affect digital communications much more severely than in the past. With the advent of digital communications, especially in connection with the global Internet, frequency errors can increase the transmission errors, usually expressed as a bit error rate (BER). Above a certain BER, digital communications become useless. At lower BERs, the systems relying on the satellite link are less efficient, less reliable and more costly. To date, however, no cost-efficient system fully compensates for these frequency translations to a sufficient degree to permit digital satellite communications to achieve their full potential and to reach the largest audience.